Ren-yong Shi

Biochar reduces the bioaccumulation of PAHs from soil to carrot (Daucus carota L.) in the rhizosphere: A mechanism study

The aim of this study was to reveal the mechanisms on how biochar reduces bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in tuberous vegetables. Corn straw-derived biochar pyrolyzed at 300°C (CB300) or bamboo-derived biochar pyrolyzed at 700°C (BB700) was amended into PAH-contaminated soil planted with carrot (Daucus carota L.). After 150days, 2% CB300 or 2% BB700 amendments significantly reduced the bioaccumulation of PAHs in carrot root (p<0.05), especially for high-molecular-weight PAHs. In the non-rhizosphere, either CB300 or BB700 suppressed PAH dissipation and decreased the bioavailability via adsorption processes. Compared to the control, the total concentration of PAHs in the rhizosphere was higher in the 2% BB700 treatment but the bioavailable concentration was lower. This indicates that BB700 decreased the bioavailability of PAHs primarily via immobilization (adsorption processes). By contrast, the total and bioavailable PAH concentrations were both lower in the 2% CB300 treatment than those in the control. The abundance of bacteria such as Arthrobacter and Flavobacterium and the total number of genes playing important roles in microbial PAH degradation processes increased significantly (p<0.05), which were likely responsible for the rapid dissipation of PAHs in the 2% CB300 treatment in the rhizosphere. These results indicate that CB300 decreased the PAH bioavailability primarily via increasing degradation of PAHs by indigenous microorganisms. The two biochars both showed better effectiveness at reducing the bioavailability of high-molecular-weight PAHs than the low-molecular-weight PAHs in the rhizosphere. Therefore, the mechanisms on how biochar reduces the PAH uptake into carrot are dependent on the type of biochar (e.g., pyrolysis temperature and feedstock) and root presence.

Mechanisms for Increasing the pH Buffering Capacity of an Acidic Ultisol by Crop Residue-Derived Biochars

The effects and underlying mechanisms of crop residue-derived biochars on the pH buffering capacity (pHbuff) of an acidic Ultisol, with low pHbuff, were investigated through indoor incubation and simulated acidification experiments. The incorporation of biochars significantly increased soil pHbuff with the magnitude of the increase dependent on acid buffering capacity of the biochar incorporated to the soil. Cation release, resulting from the protonation of carboxyl groups on biochar surfaces and the dissolution of carbonates, was the predominant mechanism responsible for the increase in soil pHbuff at pH 4.0-7.0 and accounted for >67% of the increased pHbuff. The reaction of protons with soluble silica (Si) in biochars derived from rice straw and corn stover also accounted for ∼20% of the pHbuff increase due to H3SiO4- precipitation. In conclusion, the incorporation of crop residue-derived biochars into acidic soils increased soil pHbuff with peanut stover biochar being the most effective biochar tested.

Characteristics of biomass ashes from different materials and their ameliorative effects on acid soils

The chemical characteristics, element contents, mineral compositions, and the ameliorative effects on acid soils of five biomass ashes from different materials were analyzed. The chemical properties of the ashes varied depending on the source biomass material. An increase in the concrete shuttering contents in the biomass materials led to higher alkalinity, and higher Ca and Mg levels in biomass ashes, which made them particularly good at ameliorating effects on soil acidity. However, heavy metal contents, such as Cr, Cu, and Zn in the ashes, were relatively high. The incorporation of all ashes increased soil pH, exchangeable base cations, and available phosphorus, but decreased soil exchangeable acidity. The application of the ashes from biomass materials with a high concrete shuttering content increased the soil available heavy metal contents. Therefore, the biomass ashes from wood and crop residues with low concrete contents were the better acid soil amendments.

Effects of biochars on the bioaccessibility of phenanthrene/pyrene/zinc/lead and microbial community structure in a soil under aerobic and anaerobic conditions

The immobilization of co-contaminants of organic and inorganic pollutants by biochar is an efficient remediation strategy. However, the effect of biochar amendments on the bioaccessibility of the co-contaminants in dry versus flooded soils has rarely been compared. In batch experiments, bamboo-derived biochar (BB) had a higher sorption capacity for phenanthrene (Phe)/pyrene (Pyr)/zinc (Zn) than corn straw-derived biochar (CB), while CB had a higher sorption capacity for lead (Pb) than BB. After 150days of incubation, the amendments of 2% CB, 0.5% BB and 2% BB effectively suppressed the dissipation and reduced the bioaccessibility of Phe/Pyr by 15.65%/18.02%, 17.07%/18.31% and 25.43%/27.11%, respectively, in the aerobic soils. This effectiveness was more significant than that in the anaerobic soils. The accessible Zn/Pb concentrations were also significantly lower in the aerobic soils than in the anaerobic soils, regardless of treatments. The Gram-negative bacterial biomass and the Shannon-Weaver index in the aerobic soil amended with 2% CB were the highest. The soil microbial community structure was jointly affected by changes in the bioaccessibility of the co-contaminants and the soil physiochemical properties caused by biochar amendments under the two conditions. Therefore, dry land farming may be more reliable than paddy soil cultivation at reducing the bioaccessibility of Phe/Pyr/Zn/Pb and enhancing the soil microbial diversity in the short term.

Effects of cationic surfactant on the bioaccumulation of polycyclic aromatic hydrocarbons in rice and the soil microbial community structure

Cationic surfactants have been proposed as adsorbents for the remediation of aerobic agricultural soils contaminated with polycyclic aromatic hydrocarbons (PAHs). However, the effects of cationic surfactants on the bioavailability of PAHs in paddy soils and on the soil microbial community structure have not been investigated. In this study, cetyltrimethylammonium bromide (CTMAB) was added into an anaerobic PAH-contaminated soil planted with rice (Oryza sativa L.). After harvest, the addition of 150 mg kg−1 CTMAB significantly reduced the bioavailability of PAHs in the soil and thus the bioaccumulation of PAHs from the soil into the rice roots. Based on the results of Tenax extraction, when applied at appropriate concentrations, CTMAB retarded the PAH desorption from the soil. The above effectiveness of CTMAB was better for high-molecular-weight PAHs (four to six benzene rings) than low-molecular-weight PAHs (two to three benzene rings). Amendment with CTMAB increased the bacterial diversity, species richness, and most of the bacterial phyla (especially involving Acidobacteria and Firmicutes) in the soils (p < 0.05). The contents of organic matter, ammonium nitrogen, and bioavailable PAH concentrations were the top three soil attributes that influenced the bacterial community structure in paddy soils amended with CTMAB. This study indicates that CTMAB applied at appropriate levels, can effectively reduce the environmental risk of PAHs in rice production, primarily via immobilization, and benefit the soil microbial ecology in the short term.

Incorporation of corn straw biochar inhibited the re-acidification of four acidic soils derived from different parent materials

The effect of corn straw biochar on inhibiting the re-acidification of acid soils derived from different parent materials due to increased soil pH buffering capacity (pHBC) was investigated using indoor incubation and simulated acidification experiments. The incorporation of the biochar increased the pHBC of all four soils due to the increase in soil cation exchange capacity (CEC). When 5% biochar was incorporated, the pHBC was increased by 62, 27, 32, and 24% for the Ultisols derived from Tertiary red sandstone, Quaternary red earth, granite, and the Oxisol derived from basalt, respectively. Ca(OH)2 and the biochar were added to adjust the soil pH to the same values, and then HNO3 was added to acidify these amended soils. The results of this simulated acidification indicated that the decrease in soil pH induced by HNO3 was lower for the treatments with the biochar added than that of the treatments with Ca(OH)2 added. Consequently, the biochar could inhibit the re-acidification of the amended acid soils due to the increased resistance of the soils to acidification when the pH of amended soil was higher than 5.5. The inhibiting effectiveness of the biochar on soil re-acidification was greater in the Ultisol derived from Tertiary red sandstone due to its lower clay and organic matter contents and CEC than the other three soils. The incorporation of the biochar also decreased the potentially reactive Al, i.e., exchangeable Al, organically bound Al, and sorbed hydroxyl Al, compared with the treatments amended with Ca(OH)2. Therefore, the incorporation of corn straw biochar not only inhibited the re-acidification of amended acid soils through increasing their resistance to acidification but also decreased the potential of Al toxicity generated during re-acidification.

Mechanisms of biochar reducing the bioaccumulation of PAHs in rice from soil: Degradation stimulation vs immobilization

This study aimed to elucidate the mechanisms by which biochar reduces the bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in rice under anaerobic conditions. Corn straw- or bamboo-derived biochar pyrolyzed at 300 °C and 700 °C (CB300 or BB700), respectively, was amended into flooded PAH-contaminated soil. After harvest, 2% CB300, 0.5% BB700 or 2% BB700 amendments reduced the bioaccumulation of PAHs in rice root, especially that of high-molecular-weight PAHs (p < .05). Total PAH concentrations were higher, and their bioavailable concentrations were lower in BB700-amended soils than the control. The stimulation of PAH desorption from BB by low-molecular-weight organic acids (LMWOAs) was gentle and did not significantly retard the adsorption of PAHs on BB700, indicating that BB700 reduced PAH bioavailability primarily via immobilization. The total and bioavailable concentrations of PAHs were both lower in the 2% CB300-treated soils than the control. LMWOAs facilitated PAH release from CB300-amended soils, thus increasing the bioavailability of immobilized PAHs. The relative abundances of the bacteria, functional genes, and methanogens involved in PAH anaerobic degradation were significantly higher in the 2% CB300 treatment than other treatments. Fast PAH dissipation in soil amended with 2% CB300 may be attributed to the increased bioavailability of immobilized PAHs and enhanced biodegradation, both of which were induced by LMWOAs and CB. In summary, biochar types and root presence jointly affected the mechanisms by which biochar reduced the bioaccumulation of PAHs in rice under anaerobic conditions.

Peanut straw biochar increases the resistance of two Ultisols derived from different parent materials to acidification: A mechanism study

The mechanisms for increasing soil pH buffering capacity (pHBC) and soil resistance to acidification by peanut straw biochar were investigated by undertaking indoor incubation and simulated acidification experiments using two Ultisols derived from tertiary red sandstone and quaternary red earth. The biochar increased the pHBC and resistance of the two Ultisols to acidification. The addition of 3% biochar increased the pHBC of the two Ultisols by 76% and 25%, respectively. The increased resistance of the soils to acidification led to the inhibition to decrease in soil pH and the activation of soil Al during acidification. The protonation of carboxyl groups on the biochar surface was the main mechanism responsible for resisting acidification of the Ultisols when the pH was between 4.5 and 7.0. The higher soil pH (>6.0) after biochar application and the large number of carboxyl groups on the biochar surface were essential if biochar was to significantly increase the resistance of soils to acidification.